1. Field of the Invention
The present invention relates to a signal-to-noise (Hereinafter, referred to “S/N”) enhancer, more particularly, a signal-to-noise enhancer that is implemented by using a balun coupler and a magnetostatic wave filter.
2. Description of the Prior Art
Recently, due to miniaturization of a digital broadcasting system, a satellite broadcasting system, a mobile communication system, a satellite communication system, a lightweight S/N enhancer that takes little cost is required. Also, in the characteristic aspect, the S/N enhancer having a low insertion loss, a high S/N ratio, a broadband, and low power consumption is required.
Hereinafter, a conventional S/N enhancer will be explained with reference to the accompanying drawings.
First, referring to FIG. 1, the S/N enhancer disclosed in “A Reflection type of MSW signal to noise enhancer in the 400 MHz band” of Takao Kuki and Toshihiro Nomoto, IEEE MTT-S digest vol. 41, No. 8, pp111-114, 1995 will be explained.
The S/N enhancer comprises a magnetostatic wave filter 10 and a directional coupler 12. The one end of the magneto static wave filter is connected with a circulator or a directional coupler that the input port and output port are separated. Explaining the principle thereof, when a RF signal having a small size thereof is applied to an input port, the signal is converted into a magnetostatic wave signal at an Yttrium-Iron-Garnet film. Therefore, the RF input signal is not output to the output port. Otherwise, when RF input signals equal to and more than a threshold value are applied to the input port, almost signals are reflected and output at the output port, without convert into the magnetostatic wave signals. Accordingly, the S/N enhancer that obtain a high loss when the signal has a small level and obtain a low loss when the signal has a large level be can be accomplished.
The above-mentioned S/N enhancer has merits which the structure thereof is simple and the input/output characteristics is excellent, but has demerits which the impedance matching as well as a large signal level is required.
Next, referring to FIG. 2, the S/N enhancer disclosed in “A signal to Noise Enhancer using two MSW filters and its application to Noise reduction in DBS reception” of Thoshihiro Nomoto and Yoshihiro Matsushita, IEEE Trans MTT vol. 41, No. 8, pp1316-1322, 1993. 8 will be explained.
The conventional S/N enhancer shown in FIG. 2 comprises magnetostatic wave filters 124 and 126, a phase shifter 136, an attenuator 134, and directional couplers 122 and 138. In principle, a first path signal and second path signal having different level are input to the directional coupler 122 and are distributed therein. Thereby, these two signals supplied to the magnetostatic wave filters 124 and 126, respectively. Where, while the first signal has a high level, the second signal has a low level. That is, the first signal includes a noise signal and a desired signal, wherein the noise signal passes through the magnetostatic wave filter 124, without being amplitude limited, but the desired signal is amplitude limited. In addition, the second signal has a noise signal and a desired signal which have both level lower than that of a saturation threshold power, thereby the noise level signal and the desired signal pass through the magnetostatic filter 126, without being amplitude limited.
Next, the directional coupler 138 synthesizes two path signals having the same amplitude and the opposite phase thereof with respect to the signal less than the threshold value. At the result, the noise signals are cancelled and the desired signal of the second signal becomes a main power level signal.
At this time, the level of the threshold power is in the range from −12 dBm (PH) to −19 dBm (PL), forming somewhat of a band. In addition, the attenuator 134 functions as a trimmer for compensating the power loss due to the phase shifter 136.
In the above-mentioned manner, there are merits which the input/output characteristics thereof is excellent and it is advantageous in the insertion loss, but there are demerits which it is can be not used at the low power.